Complete device layer transfer without edge exclusion via direct wafer bonding and constrained bond-strengthening process

ABSTRACT

More complete bonding of wafers may be achieved out to the edge regions of the wafer by constrained bond strengthening of the wafers in a pressure bonding apparatus after direct wafer bonding. The pressure bonding process may be accompanied by the application of not above room temperature.

BACKGROUND

[0001] This invention relates generally to wafer bonding.

[0002] In wafer bonding, two semiconductor wafers may be placed in aface-to-face configuration. A layer on one semiconductor wafer may betransferred to the other semiconductor wafer in a process called waferbonding. A wide variety of layers may be transferred betweensemiconductor wafers. One application for wafer bonding is in connectionwith forming silicon on insulator (SOI) devices.

[0003] Generally, a pair of opposed flat silicon wafers are contacted toone another so that they physically and chemically bond. A layer istransferred from a donor wafer to a handle wafer.

[0004] One problem with existing wafer bonding processes is that aperipheral region of the handle wafer, generally about 3 to 5millimeters, may remain unbonded. This unbonded peripheral region is aregion on the outer periphery of the wafer extending radially inwardlyfrom the edge of the wafer to a distance of about 3 to 5 millimeters.

[0005] As a result of this unbonded region, islands of material, debris,particles, and flakes may collect in the unbonded region created by theresulting edge. These particles may ultimately release, resulting inproblematic defects. In addition, the wafers may only have a useablesurface area up to 3 to 5 millimeters inwardly of the outermost edge.The unbonded area may result in some loss of useable wafer area.

[0006] Thus, there is a need for better ways to wafer bond wafers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is a cross-sectional view of one embodiment of the presentinvention;

[0008]FIG. 2 is a cross-sectional view of wafers in accordance with oneembodiment of the present invention; and

[0009]FIG. 3 is a partial cross-sectional view of the results of waferbonding in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION

[0010] Referring to FIG. 1, a pair of wafers 12 a and 12 b may belocated in a free state condition on a bonding plate 20. Bonding may beinitiated at the edge or center of the wafers 12.

[0011] During bonding, elastic deformation of donor wafer 12 b andhandle wafer 12 a may occur microscopically, compensating for surfaceroughness, topography, flatness, and profile in the center and edgeregions of the wafers. The strength of the surface Van der Waals forcesmay not be sufficient to elastically deform the areas at the edges ofthe wafers 12.

[0012] Even where bonding occurs at the edges of the wafers 12, thebonding forces may not be strong enough to overcome the natural tendencyfor the wafers to pull apart due to the surface characteristics. Thisleads to non-layer transfer areas resulting in loss of transferreddevice film of up to 5 millimeters unbonded inboard of the circular areaat the handle wafer edge.

[0013] Thus, as shown in FIG. 1, the wafers 12 may be pressed togetherto direct bond and transfer the film 14 from the donor wafer 12 b to thehandle wafer 12 a. The donor wafer 12 b may be mounted on a mountingplate 20. The wafers 12 a and 12 b may be pressed together centrally orperipherally as indicated by the pressure element 16 and the associatedarrow. At such time, the wafers 12 may be held in alignment by the jig18.

[0014] After direct bonding, the bonded pair may be put into a pressurebonding apparatus, shown in FIG. 2, to flatten and bring into contactunbonded areas that split from the initial bonding of the handle anddonor wafers 12. In this case, a pair of rigid, flat, parallel plates 22a and 22 b may be positioned on either opposed surface of the bondedwafers 12 and pressure may be applied substantially uniformly across atleast one plate 22 while the other plate 12 is supported. In oneembodiment, the applied pressure may be from 0.01 pounds per square inchto 0.35 pounds per square inch. The pressure may be applied for 10 to 30minutes in some embodiments.

[0015] Bond strengthening may be achieved by heat treatment of thebonded pair in the pressure bonding apparatus shown in FIG. 2. The heattreatment may convert Van der Waals surface interactions into strongercovalent bonds between donor and handle wafers 12 over the entire wafercontact area.

[0016] Then subsequent layer exfoliation results in more complete devicelayer transfer as shown in FIG. 3. As indicated in FIG. 3, the film 14from the donor wafer 12 b may be transferred close to the peripheraledge 24 of the wafer 12 a. In one embodiment, the wafer 12 a may be asilicon on insulator wafer having bulk silicon 28 covered by aninsulator 30 over which is bonded the film 14.

[0017] The heat processing may involve temperatures of 100 to 600° C.for times from 1 to 30 minutes in some embodiments of the presentinvention.

[0018] As a result, in some embodiments, even where-wafernon-uniformities occur, direct wafer bonding of donor and handle wafersaccompanied by constrained annealing of the bonded pair facilitatecomplete wafer bonding. As a result, the 3 to 5 millimeter region ofnon-bonding with conventional processes may be reduced, facilitatingcomplete wafer surface bonding. In some embodiments, less than 3millimeters of edge exclusion 26 may occur with complete surface areacontact and film 14 bonding across the wafer 12 a.

[0019] This more complete bonding may reduce the edge region that tendsto collect particles and flakes. This may reduce the ensuing defectscaused by such particles in some embodiments.

[0020] While the present invention has been described with respect to alimited number of embodiments, those skilled in the art will appreciatenumerous modifications and variations therefrom. It is intended that theappended claims cover all such modifications and variations as fallwithin the true spirit and scope of this present invention.

What is claimed is:
 1. A method comprising: direct contact bonding apair of wafers by the application of pressure; and subjecting the pairof wafers to constrained wafer bonding at temperatures above roomtemperature.
 2. The method of claim 1 including direct contact bondingby applying peripheral pressure to bond the wafers.
 3. The method ofclaim 1 wherein subjecting includes constraining the pair of wafersbetween a pair of rigid, parallel plates.
 4. The method of claim 3including applying heat of approximately 100 to 600° C. while saidwafers are between said plates.
 5. The method of claim 4 includingapplying heat for at least one minute.
 6. The method of claim 3including converting Van der Waal surface interactions into strongercovalent bonds between the wafers by the application of heat.
 7. Themethod of claim 1 including creating an unbonded surface area of lessthan 5 millimeters from the peripheral edge of a wafer.
 8. The method ofclaim 1 wherein subjecting includes applying substantially uniformpressure across the entire surfaces of said wafers.
 9. The method ofclaim 8 including applying pressure to said wafers of at least 0.01pounds per square inch.
 10. The method of claim 9 including applying apressure less than about 0.35 pounds per square inch.
 11. Asemiconductor wafer comprising: bulk silicon material; and a waferbonded surface film extending across said bulk silicon material towithin 5 millimeters of the peripheral edge of said bulk silicon. 12.The wafer of claim 11 wherein said wafer is a silicon on insulatorwafer.
 13. A method comprising: positioning a pair of wafers inface-to-face contact; applying pressure to said wafers to initiate waferbonding; after applying pressure, positioning the wafers between a pairof parallel rigid plates; pressing said wafers together between saidplates; and heating said wafers while said wafers are between said pairof parallel plates.
 14. The method of claim 13 including applying heatwhile said wafers are between said plates of approximately 100 to 600°C.
 15. The method of claim 14 including applying heat for at least oneminute.
 16. The method of claim 15 including creating an unbondedsurface area of less than 5 millimeters from the peripheral edge of awafer.
 17. The method of claim 13 including pressing by applyingsubstantially uniform pressure across the entire surface of a waferthrough said parallel plates.
 18. The method of claim 17 includingapplying pressure to said wafers of at least 0.01 pounds per squareinch.
 19. The method of claim 18 including applying pressure less thanabout 0.35 pounds per square inch.
 20. The method of claim 13 includingconverting Van der Waal surface interactions into stronger covalentbonds between the wafers by the application of heat.